Metal halide high-pressure discharge lamps of low power, for example about 50 W or less, have been proposed for use in automotive vehicles. Such lamps are operated at, for example, about 100 V, at frequencies for example about 45 kHz. The operating power is derived from an inverter circuit which, in turn, is energized from an automotive vehicle battery. The referenced application Ser. No. 07/452,125, filed Dec. 15, 1989, assigned to the assignee of the present application, describes such a lamp-circuit combination.
Lamps of the type suitable for use in automotive vehicles previously have been made by first closing an open quartz tube. The generally olive-shaped bulb of the discharge vessel was then formed. The originally closed end was then opened, and an exhaust tube was secured to the discharge vessel approximately at the center thereof. Electrode systems were introduced into the open ends, melted and sealed therein, for example by standard press or pinch seals. Then fills and a fill gas were introduced through the exhaust tube into the discharge vessel, and the exhaust tube was then tipped off. This method is complex, utilizes a number of steps which are difficult to automate, and has the substantial disadvantage that the discharge vessel, which is already tiny, may be subject to non-homogeneities in the material distribution. A typical dimension of the discharge vessel is a length of about 7.5 mm, a diameter of only about 5.5 mm. Attaching, and later on tipping off the exhaust tube may change the distribution of the material so that cold spot temperatures may arise at undesired points during operation of the lamp. Such uncontrolled location and temperature of cold spots can change the color temperature of the emitted light uncontrollably and detrimentally. Further, the light derived from the lamp is difficult to control, so that stray and spread radiation may result. When such lamps are then combined with optical systems such as reflectors, lenses and the like, the light distribution may not be as desired and in accordance with automotive and governmental standards.